I read an article attempting to explain why
viruses attack cells. The argument mentioned by
the author hinges around the idea that a virus is
genetically programmed to attack cells. We
already know that. What is missing is how this
viral genetic code was written in the first place.
This is not a philosophical question!

The point is as follows: evolution is an
ongoing biological process and different life
forms are different form one another. Having said
that, it is not clear how the genetic make-up of a
virus can be designed in order to successfully
attack cells. Consider the differences between a
bacteriophage and the Human Immunodeficiency Virus
(HIV). Clearly, they have different structural
properties that allow them to successfully invade
the corresponding targets. Why? How did it
happen? What are the fundamental mechanisms
underlying viruses, i.e., design and build a virus
that will be able to infect the following cell?
Scientific answers are required to tackle these
hard questions.

Question Date: 2013-10-31

Answer 1:

This is an interesting question and in fact, the
origins of viruses, as well as life in general,
are something that scientists are still trying to
determine. There are ideas about how certain
"characteristics" are incorporated into the
structure of viruses (and living organisms!), such
as an iterative process that basically looks like:

1) mutation and crossing over between parents
leads to higher viability of the organism
2) thus these genes get passed on to a next generation
3) repeat 1) and 2).

Through this iterative process, the "fitness
landscape" of a species is explored, and
ultimately, the individuals at the local maxima
(peaks) of the fitness landscape, who are more
"fit" to survive are the ones whose traits get
passed on. It just happens that the
characteristics that resulted in higher fitness in
individuals (i.e. viability) might be different
from one kind of organism to another because of
the randomness that went into the iterations.
Inherently, a mutation or a crossing over event
during reproduction is random.

Answer 2:

Viruses are contained within protein sheathes
when not in cells. The sheath of each virus is
different. Each virus' protein sheath both enables
it to recognize cells that it can infect and
physically inserts the viral DNA/RNA into the
cell(s) that the virus does infect. How it does
this is going to depend on the cell in question
and presumably by the proteins or other chemicals
embedded within the membrane of the cell the virus
is going to infect. Not being a microbiologist, I
can't tell you the specifics of how any one virus
manages to infect its host cell, but I can tell
you that a virus' ability to do so is maintained
by natural selection. I'm also sure that the
fundamental means by which viruses can infect
cells varies considerably from one virus to the
next, and is an area of ongoing research (in other
words, we don't have all of the answers yet).

Where viruses come from is another question
where we have only partial answers. As far as I
know, most viruses are thought to be derived from
the organisms that they infect; they did not
create other forms of life. There are bits of
free-floating DNA in some cells, especially
bacteria (they're known as plasmids), but they
aren't encased in protein sheaths like viruses.
How that happens I certainly don't know and I
doubt anybody.